(a) Field of the Invention
The present invention relates to a wet type electrostatic precipitator, and more particularly to a wet type electrostatic precipitator having a discharge electrode arranged along a flow path of a gas to be treated.
(b) Description of the Related Arts
Exhaust gas exhausted from a furnace that employs heavy oil or coal as a fuel contains sulfur oxides generated from sulfur contents in the fuel. Therefore, from such exhaust gas exhausted from a boiler for a thermal power plant, dust is removed by a dry-type electrostatic precipitator, then, sulfur oxides are removed by a wet type desulfurizer, and finally, the exhaust gas is guided to a wet type electrostatic precipitator where a mist or the like is removed and then the resultant is emitted to the atmosphere, as disclosed in the Japanese Unexamined Patent Application No. 2002-45643.
Although the sulfur oxide contained in this type of the exhaust gas is mainly sulfur dioxide, sulfur trioxide of several ppm level is present. The sulfur trioxide reacts readily with water to become sulfuric acid, which is condensed to form a sulfuric acid mist when a gas temperature becomes the dew point of sulfuric acid or lower. Since the sulfuric acid mist has strong corrosivity, the temperature of the exhaust gas is kept to the temperature higher than the dew point of the sulfuric acid (e.g., about 170° C.), at a stage before the wet type desulfurizer. However, when the exhaust gas is guided to the wet type desulfurizer to rapidly cool the same to about 55° C., which is the dew point of water, a micromist of sulfuric acid is formed. The micromist is difficult to be removed by the wet type desulfurizer, so it is removed at the wet type electrostatic precipitator at a later stage.
In the wet type electrostatic precipitator, a mist such as the micromist of sulfuric acid in the exhaust gas transmitted from the wet type desulfurizer and remaining dust is collected to a dust-collecting electrode based on a principle of electrostatic precipitation. The collected mist itself forms a wet film on the surface of the dust-collecting electrode and naturally flows down. When the amount of the mist is small and the natural flow-down is difficult to occur, washwater is flown all times or intermittently from above the dust-collecting electrode so as to flow down the mist and dust collected on the dust-collecting electrode.
However, when the gas to be treated containing a corrosive mist, such as sulfuric acid mist treated in the wet type electrostatic precipitator, a mist is collected on the dust-collecting electrode, whereby the discharge electrode is likely to be dried. Therefore, when the corrosive mist in the gas to be treated is adhered onto the discharge electrode, the corrosive mist is enriched due to the dryness. Accordingly, the discharge electrode is corroded, thereby entailing a problem of decreasing the usable life of the discharge electrode. In order to improve the problem described above, it is considered that washwater is sprayed from above the discharge electrode to flow down the corrosive mist adhered onto the discharge electrode. However, in such way, the sprayed water droplets are flown by a gas flow, and thus the water droplets cannot reach the lower part of the discharge electrode. Therefore, it is difficult to flow down the corrosive mist adhered onto the discharge electrode entirely. Further, if the particle diameter of the sprayed water droplet is increased so as to prevent the water droplet from being flown by the gas flow, most of the water droplets are collected to the dust-collecting electrode. Therefore, sufficient washing effect cannot be attained, and conversely, there arises a problem that the water droplets induce sparks.